This application is based on an application Ser. No. 11-138207 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a low pressure mercury vapor discharge lamp that includes U-shaped glass tubes filled with mercury vapor.
(2) Prior Art
One kind of well-known low pressure mercury vapor discharge lamp includes an arc tube that is a plurality of connected glass tubes. In the arc tube, one discharge path is formed. This arc tube is enclosed in a glass globe or a plastic globe, and a lighting circuit is enclosed in a case with a lamp cap. A lamp whose housing is composed of the globe and the case is known as the bulb-shaped fluorescent lamp.
This fluorescent lamp is desired to be more compact so as to be small as the bulb. In addition, the fluorescent lamp is desired to be more efficient and have a longer life.
In consideration of the size and the maximum luminous efficiency of the fluorescent lamp, however, the glass tube diameter, the discharge path length, and the inter-electrode distance of the arc tube cannot be increased any more. Under the circumstances, the arc tube is desired to still improve the luminous efficiency.
It is accordingly an object of the present invention to provide a low pressure mercury vapor discharge lamp having improved luminous efficiency without upsizing the arc tube.
Another object of the present invention is to improve the luminous efficiency by devising the shape of the bent part of the U-shaped glass tube without upsizing the arc tube.
A further object of the present invention is to provide a low pressure mercury vapor discharge lamp that is preferable as a bulb-shaped fluorescent lamp.
Yet another object of the present invention is to provide a rational method for manufacturing a low pressure mercury vapor discharge lamp so as not to upsizing the arc tube and to improve the luminous efficiency.
The above-mentioned first and second objects may be achieved by a low pressure mercury vapor discharge lamp having a U-shaped glass tube characterized in that a part of an outline of a sectional face at a bent part of the U-shaped glass tube is a gentle curve.
In the low pressure mercury vapor discharge lamp, the inter-electrode distance, i.e., the discharge distance, which is one factor for the luminous efficiency, can be increased. As a result, the luminous efficiency can be improved without increasing the entire length of the arc tube.
The above-mentioned first and second objects may be also achieved by the low pressure mercury vapor discharge lamp, wherein the gentle curve is formed on a surface on an inner curve side of the U shape of the U-shaped glass tube.
The above-mentioned first and second objects may be also achieved by the low pressure mercury vapor discharge lamp, wherein a remaining part of the outline is a circular arc.
The above-mentioned first and second objects may be also achieved by the low pressure mercury vapor discharge lamp, wherein the gentle curve is approximately a chord.
The above-mentioned first and second objects may be also achieved by the low pressure mercury vapor discharge lamp, wherein the gentle curve satisfies an equation 0.5 less than d1/Dxe2x89xa60.9, a shortest internal diameter of the U-shaped glass tube at the bent part being d1, and an internal diameter of the U-shaped glass tube at a straight part being D.
The above-mentioned first and second objects may be also achieved by the low pressure mercury vapor discharge lamp, wherein the part of the sectional face outline at the bent part of the U-shaped glass tube on the surface on the inner curve side of the U shape of the U-shaped glass tube is a gentle curve.
The above-mentioned third object may be also achieved by the low pressure mercury vapor discharge lamp, wherein the U-shaped glass tubes have the same sectional face outline at bent parts.
The above-mentioned fourth object may be also achieved by a low pressure mercury vapor discharge lamp that includes a plurality of U-shaped glass tubes, wherein the U-shaped glass tubes are connected to each other so as to form a discharge path inside, and a part of an outline of a sectional face at a bent part of at least one of the U-shaped glass tubes is a gentle curve; a holder for holding an arc tube; a lighting circuit for applying lighting voltage to the arc tube held by the holder between ends of the discharge path; and a lamp cap that is integrated with the holder, the lamp cap connecting the lighting circuit to an external power.